The use of electrical discharge machining apparatus for producing tapped holes in a workpiece is well known in the art. EDM's (electrical discharge machines) are commonly employed for this purpose where the workpiece comprises high strength, very hard steel or other electrically conductive material.
The electrical discharge machining process is based on the principle of erosion of the metal workpiece by spark discharges. The spark is a transient electrical discharge through the space between two charged electrodes, which are the tool and the workpiece. The discharge occurs when the potential difference between the tool and the workpiece is large enough to cause a breakdown in the dielectric fluid (usually oil) in which the workpiece and the tool are immersed, and to procure the electrically conductive spark channel. The breakdown potential is establshed by connecting the two electrodes to the terminal of a capacitor charged from a power source. The spacing between the tool and the workpiece is critical. Consequently, the feed rate sometimes is controlled by several mechanisms. The dielectric fluid in which the spark gap between the workpiece and the tool is immersed provides the additional function of cooling the area surrounding the spark gap and carrying away machined particles produced by the discharge.
The rate of metal removal depends on several variables, including the average current in the discharge circuit, the electrode characteristics and the nature of the dielectric fluid. Removal rate also depends on the frequency of arcing which often occurs between the electrodes when the electrodes are drawn too close together; during an arcing condition, material removal becomes unpredictable and the workpiece may become overheated or pitted. Consequently, in the event of an arc between the electrodes, it becomes necessary to increase the spacing between the electrodes; this is normally accomplished by reversing the travel of the tool until the arc is terminated following which the tool is again advanced toward the workpiece at a prescribed rate.
One known prior art device for producing tapped holes in a workpiece consists of a hand-operated assembly in which the tool, which is provided with a thread from thereon, is advanced by a hand-operated wheel. Naturally, it is extremely difficult for a human to advance the tool at a constant, controlled rate of speed. Moreover, a considerable amount of time is required for the operator to detect an arcing condition and reverse the direction of the tool. Consequently, these hand-operated devices are not only inefficient in terms of the time required to tap a particular hole in a workpiece, but also result in an inferior quality thread in the tapped hole.
Other types of mechanisms employed for producing tapped holes utilized hydraulically-operated servo-control mechanisms and a hydraulic motor for rotating and advancing the tool. Typically, the hydraulic motor is mounted directly on the vertically shiftable machining hand and the workpiece is positioned on a bed or table immediately beneath the head. The head is then lowered at a prescribed rate while the hydraulic motor turns the tool. A lead thread positioned at the side of the tool advances at the same rate as the tool, thereby to provide the operator with an indication of the feed rate. The tool is guided toward a preselected location on the workpiece by means of a threaded guide or pilot which is directly secured to the workpiece itself.
The device described immediately above is less than completely satisfactory for several reasons. First, the threaded guide surrounds the hole being tapped and therefore substantially impedes the escape of fluid flowing through the tool into the electrode gap which carries away machined particles. Another problem related to this prior device involves the fact that a certain amount of tolerance exists between the male threads of the tool and the female threads of the threaded guide; when the motor is reversed to eliminate an arcing condition, the threads shift relative to each other to one end of the tolerance range, thus altering the electrode spacing by the degree of shift. However, when the arcing condition ceases and the motor is once again reversed to advance the tool, the threads shift again to the other range of the tolerance range thereby closing the gap between the electrodes more than intended.
It is therefore a primary object of the prevent invention to overcome each of the deficiencies inherent in prior art devices of the type discussed above.